Colored coatings are known in the art. For example WO 03/023816 and WO 04/044487 disclose lamp vessels provided with a light absorbing coating.
When a coating is applied on a substrate, said substrate may not be perfectly clean. The presence of particles, even of chemically inert particles, creates some very localized stresses around said particles and leads to micro-cracks, which can rapidly spread throughout the coating. Some sol-gel-containing coatings can handle particle-induced stress without crack spreading. In the industry, a wide range of elements can contaminate substrates. In the presence of such contaminants on a substrate, adhesion between coating and substrate is dramatically decreased. Contaminants may also create additional stresses which cannot be handled by conventional sol-gel coatings. These effects can lead to macro-cracks and peeling off, particularly when said coating is subjected to temperature variations. In addition, many contaminants (for example Zn, Cl, Sn, Na, Ca, W, glycerin or organic resins like rosin/colophony) are not chemically inert with respect to the substrate and/or the coating. They induce new physical and/or chemical mechanisms and phenomena, which lead to additional stresses and poor adhesion. For instance, said contaminants can locally modify the pH of a coating, induce modifications of matrix structure, or simply react with compounds of the coating itself, modify the stochiometry required for proper matrix formation. As the coating is locally modified, its structure is not homogeneous and additional stresses are generated.
Moreover, some substrates exhibit a relatively low —OH and —O. density on their surface. In some cases, this is due to a prior surface treatment. Such a low —OH and —O. density generally does not favour coating adhesion.